The purpose of wastewater treatment is generally to remove from the wastewater enough solids to permit the remainder to be discharged to a receiving water source (river, holding pond, urban drainage etc.) without interfering with its proper use and often, within analytical guidelines set by a controlling agency. Wastewater may be treated using physical, chemical or biological processes or a combination of these. The preliminary treatment regimen in most wastewater treatment processes involves a screening procedure to remove coarse materials (paper, sticks, cans etc.) followed by the removal of heavy inorganic particles (sand, pebbles, etc.) by a settlement process.
One common chemical treatment involves the use of ozone. Consider background patents which illustrate, for example, the large number of water purification ozonization techniques such as in U.S. Pat. No. 5,178,755, issued to LaCrosse, that discloses a method for treating wastewater, that has been enhanced by treatment with ultra-violet light and with ozone. In this system, a large amount of ozone is generated and inserted at several points in the effluent flow, including insertion in each of the three clarifiers. This system utilizes large quantities of ozone at a relatively high cost and low efficiency. Furthermore, in this system; water is continually re-circulated based upon a timer and the system does not automatically respond to changes in the influent quality or discharge water from the system based upon water quality parameters.
Another example, U.S. Pat. No. 4,798,669, issued to Bachhofer, et al., discloses an apparatus for mixing the ozone with water which is then trickled over packing material to entrain the ozone within the water being treated. The water is re-circulated through a return branch and mixes with the incoming contaminant water before it enters the treatment system. The re-circulation step is not automatic or self-adjusting based upon water quality parameters. Further ozone contact with the contaminants, after filtration of the effluent, is made by inserting recaptured gas into the effluent stream prior to the effluent being treated by a packed column which trickles the effluent over a packing material. As agitation reduces the capability of water to retain ozone, trickling over packing material may be detrimental to the efficient retention of ozone within the effluent for effective contact with contaminants. See also, U.S. Pat. No. 5,466,374 to Bachhofer et al., U.S. Pat. No. 5,207,993 to Burris, U.S. Pat. No. 3,945,918, issued to Kirk; and U.S. Pat. No. 5,273,664, issued to Schulz for additional configurations used for ozone treatment.
Probably the most commonly used chemical process is chlorine treatment. Like ozone, chlorine is a strong oxidizing agent used to kill bacteria and to slow down the decomposition of the wastewater. Even in systems involving biological treatment of wastewater, chlorine is often added in a penultimate step prior to discharge in order to remove specific organisms which are responsible for infectious diseases including, for example, Shigella Dysenteriae, Salmonella, Escherichia Coli, Klebsiella Pneumoniae, Vibrio Cholerae, Giardia spp., Cryptosporidium spp. and Entamoeba Histolytica. Residual chlorine is neutralized with sulfur dioxide before final discharge.
Nitrogen removal from wastewater can be accomplished through various chemical, physical and biological processes. In contrast to chemical treatments which tend to destroy microorganisms, biological treatment methods use microorganisms, mostly bacteria, in the biological decomposition of wastewaters to stable end products. More microorganisms are produced, and a portion of the waste is converted into carbon dioxide, water and other end products of microbial metabolism.
Nitrogen and nitrogenous compounds are often the focal point for biological wastewater treatment. Biological processes are the most common methods encountered in wastewater treatment due to the low capital costs, low operating costs, and high rates of nitrogen removal. Although nitrogen can be found in wastewater as nitrogenous organic compounds such as proteins and amino acids (Metcalf et al 2003), nitrogen is most commonly found in the forms of ammonia (NH3), ammonium (NH4+), nitrite (NO2−), and nitrate (NO3−). Biological nitrogen removal is achieved by two processes, nitrification and denitrification. The first step in nitrogen removal is biological nitrification, which is the oxidation of ammonium (NH4+) to nitrite (NO2−) and then nitrite is further oxidized to nitrate (NO3−). The second step in nitrogen removal is biological denitrification. Denitrification is a multi-step process that reduces nitrate to nitrogen gas (N2). Nitrogen gas is then released into the atmosphere and thus completes the biological nitrogen removal process from wastewater.
Soluble organic nitrogenous compounds are quickly converted to ammonium through a biological-mediated process. Ammonium is the chemical of interest when nitrogen removal is required in a treatment process. There are many different forms of human activity that generate wastewater with large quantities of ammonium: petrochemical, pharmaceutical, fertilizer and food industries and leachates produced by urban solid waste disposal sites and waste from their agricultural equivalents (pig farms etc.). (Carrera, et al 2003). When present in water, ammonia will dissociate into both ammonium ions (NH4+) and un-ionized ammonia (NH3).NH3+H2O←−→NH4++OH−  Equation 1
The concentration of un-ionized ammonia increase with increasing pH and increasing temperature (Viessman, et al., 2005). Wastewater is slightly alkaline with a pH in the range of 7.0-8.0; therefore ammonium is the dominant species in wastewater.
To remove potentially toxic nitrogenous compounds from wastewater, biological nutrient removal (BNR) is the most common method utilized. BNR for nitrogen is a two step activated sludge process that converts ammonium to nitrogen gas through nitrification and denitrification. Nitrification is the biological oxidation of ammonia to nitrate. It is an aerobic, two step process which proceeds as a result of metabolism by autotrophic, nitrogen-oxidative bacteria species known as Nitrosomas and Nitrobacter. Autotrophic bacteria require oxygen both for respiration and chemical oxidation purposes. As their sources of energy, they utilize inorganic compounds and require either carbon dioxide, bicarbonate or carbonate for their carbon source used in cell synthesis.
Nitrification can be summarized by the following three equations:NH4++1.5O2→NO2−+2H++H2O+biomass energy  Equation 2NO2−+0.5O2→NO3−+biomass energy  Equation 3NH4++2.0O2→NO3−+2H++H2O+biomass energy  Equation 4
The ammonia oxidation shown in Equation 2 is accomplished by the autotroph Nitrosomas. Ammonia serves as an electron donor while molecular oxygen serves as the electron acceptor. Ammonia is oxidized to nitrite (NO2—) in a step-wise process via hydroxylamine (NH2OH) through the use of the enzyme ammonia monoxygenase as seen in Equation 5. Equation 6 illustrates the enzyme hydroxylamine oxidoreductase converting hydroxylamine to nitrite.NH3+O2+2H+→NH2OH+H2O  Equation 5NH2OH+H2O→NO2−+5H+  Equation 6In Equation 3, nitrite is further oxidized to nitrate. This process is carried out by the autotrophic nitrite oxidizing bacteria known as Nitrobacter. Nitrobacter can grow heterotrophically in the presence of acetate, formate or pyruvate and will utilize the enzyme nitrite oxidoreductase for nitrite reduction as seen in Equation 7. (Bitton, 2005).NO2−+0.5O2→NO3−  Equation 7
Nitrification is an energy yielding process and the energy generated is used to assimilate elementary carbon sources like carbon dioxide, bicarbonate and carbonate to meet the carbon requirements for reproduction of the nitrifying bacteria. Being an aerobic process, the oxygen requirement is 4.6 mg O2/mg ammonia for complete oxidation to nitrate. Sufficient alkalinity is also needed as a buffering agent for the nitrous acid, HNO2 (H++NO2−) produced during the nitrification and is consumed at a rate of 15.7 kg CaCO3/kg NH3 oxidized.
Presently there exist a number of types of water purification devices designed to remove nitrogen from wastewater through biological processes. A traditional means involves treating the wastewater with activated sludge. Activated sludge is defined as sewage mixed with bacteria and protozoa that thrive in and multiply in it and lead to its oxidation. Activation reduces the organic pollution that raw sewage otherwise would impose on a body of water to which it is discharged.
Biological nitrogen removal is achieved through the use of either suspended growth treatment or attached growth treatment. Suspended growth is the utilization of a suspended activated sludge containing aerobic autotrophic bacteria for nitrification in an aerated tank and facultative heterotrophic bacteria for denitrification in an anoxic setting. Attached growth is the use of similar bacteria as in a suspended growth reactor, but the bacteria forms a layer of growth, known as a biofilm or fixed film, on a specific fixed film media. Submerging attached growth media in a suspended growth aeration tank, thus combining these technologies, is commonly known as an integrated fixed film activated sludge system (IFAS).
Systems utilizing submerged growth media include floating media and fixed media. Examples of floating media include plastic balls (Stuth U.S. Pat. No. 5,609,754), polyurethane foams (Reimann et al U.S. Pat. No. 4,500,429, Fuchs U.S. Pat. No. 4,415,454 and Reimann U.S. Pat. No. 4,566,971 and polyethylene pellets (Malone U.S. Pat. No. 5,126,042).
Fixed media systems are more varied. Simple systems have been described which have plain flat surfaces. Systems which are designed for having larger exposed surface areas include packed corrugated panels (Volland U.S. Pat. No. 5,545,327, Cox U.S. Pat. No. 6,942,788) or pipes. These are generally aerated systems which use variously positioned air release mechanisms to drive and direct the flow through the media configuration. Another configuration described by Gothreaux (U.S. Pat. No. 6,207,047) involves a porous grid as a support media for bacterial growth and is purported to allow for larger volume wastewater treatment
A number of anaerobic treatment systems have been described which involve a fixed medium which forms a packed bed through which the wastewater is passed. Such systems suffer from the disadvantage of gradually reducing flow rates due to accumulation of solids within the bed. Crawford et al (U.S. Pat. No. 4,676,906 and U.S. Pat. No. 4,780,198) describe a hybrid high rate process which uses a sludge blanket with a filter bed which retains biosolids within the digestor. Aerated fluidized bed or suspended bed systems have also been described (Reimann U.S. Pat. No. 4,566,971).
In what might best be described as a moving fixed media system, U.S. Pat. No. 4,093,539 to Guarino describes a system wherein the media is comprised of a partially submerged rotating contactor located within an aeration tank. The rotation of the media is driven by a supplemental air source.
A major disadvantage encountered with each of the fixed media systems described within the prior art is the exposure of the biofilms to turbulence within the wastewater treatment apparatus. In fact, a number of these systems were designed specifically to expose the media surfaces to turbulence as demonstrated by the rotating media described by Guarino and those wherein aeration is directed over the media such as that described by Volland (U.S. Pat. No. 5,545,327), Cox (U.S. Pat. No. 6,942,788) and Reimann (U.S. Pat. No. 4,566,971). Both forced motion or the bubbling of gases are sources of extreme turbulence. Even in systems where turbulence is not designed-in, exposed media surfaces are subject to fluid motions within the tank due to the pumps used for intertank wastewater exchange. Such turbulence can adversely affect the growth and ultimate thickness of the biofilms by causing these delicate structures to slough off. This loss of biofilms through sloughing off lowers the overall efficiency of the wastewater processing apparatus and creates a further issue of removal of the resulting solids from the wastewater. Since sloughing of the biofilms from fixed film media affects the quality of the effluent, the placement of such systems is generally limited to the aeration section of the wastewater treatment apparatus (4 in FIG. 1). Placement of unprotected media within the clarifier section (6 in FIG. 1) is greatly restricted since, for effluent arising from this section, quality is paramount. In as much as the art consists of various types of fixed film media configurations, it can be appreciated that there is a continuing need for and interest in improvements to these purification systems and their configurations, and in this respect, the present invention addresses these needs and interests.